Friday, May 6, 2011

A how to guide for the Power Circuit of a 3 Phase Direct On Line DOL Electric Motor Controller

In the simplest method of motor operation for small size AC induction motors, the Direct On Line (DOL) motor controller is most popular in terms of simplicity and control device availability. DOL motor controller is suitable for application with small capacity motors which does not cause too much high inrush current that can pull down the supply voltage of the available power source from the electricity supply grid.


DOL does not necessarily need complex switching devices to run the motor. The electrical diagram below shows a typical 3 phase DOL motor control power circuit diagram used in every electrical industry involving industrial process automation control technology.
wiring diagram DOL electric motor control power circuit
DOL Power Circuit

The DOL motor starter consists of the most basic electrical components for motor operation, these components includes switches and motor circuit fault detection devices.

The main circuit breaker serves as the main connecting switch for the main supply source voltage to power the whole motor control system. The circuit breaker is also fitted with overload and short circuit protection to detect faults in the load circuit, which when detected would instantly disconnect the system from the main power source to protect the motor circuit.

The main magnetic contactor serves as the motor operation switch which can be turned ON and turned OFF to connect or disconnect power to the motor which results in the running and stopping of the electric motor.

The thermal overload relay acts as over current protection that switches OFF the entire control circuit to prevent burning of the motor upon detection of high load current that exceeds the motor's rated capacity as a direct result of excessive load driven by the motor .

The DOL power circuit illustrated here is controlled by a separate control circuit. The DOL control circuit activates the closing and opening of the main contactor of the power circuit to run or stop the motor.

The DOL power circuit discussed above is controlled by a corresponding DOL motor control circuit presented in the next article.

To facilitate for a further understanding about the concept of the DOL electric motor control circuit, a flowchart diagram is provided to serve as a schematic representation of the sequence of operation of the DOL electric motor control circuit.

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